I've covered the trombone part for years in small jazz bands. I've used a marching french horn, a mellophone and a marching baritone. I've found the closest is a marching baritone. There is also a marching trombone, also called a flugabone. They are costly, but so is a quality valve trombone. I know a local trumpet player who has been doing the valve trombone for years in a local big band. It works. And you can immediately step in and play.
Richard
King 1130 Flugabone
King 2280 Euphonium
King 10J Tuba
Conn 22B Trumpet